An alternative way to test a woman’s egg for chromosome abnormalities is being developed by scientists. This new technique can enable doctors to avoid the need to manipulate and biopsy the egg itself. The research can also provide significant new information about the critical role played by some genes in the development of chromosome anomalies. Abnormalities in the chromosomes of an embryo are a leading cause of miscarriages and disorders such as Down’s syndrome.

During the preimplantation genetic screening (PGS) also known as ’embryo screening’, which is a routine procedure conducted at fertility clinics, the doctors try to hand pick an egg or an embryo that is free from any abnormality, e.g. an extra copy of chromosomes etc. Presently, to be able to correctly identify the egg or embryo, either a portion of the egg referred to as the polar body is biopsied, or a cell is extracted from an embryo for examination. In addition to being costly and invasive, these procedures carry a high risk as they can cause harm to the egg or embryo.

Dr Elpida Fragouli, and colleagues have discovered that cells surrounding an egg can provide vital information about its genetic and chromosomal status. This was revealed by Dr. Fragouli on July 6, 2011 at the annual meeting of the European Society of Human Reproduction and Embryology. She serves as a research scientist at the University of Oxford and is the director of cytogenetics at Reprogenetics UK.

Fragouli stated:

“In the ovary, human eggs are surrounded by a cloud of tiny cells, known as cumulus cells. The egg and the cumulus cells are in constant communication and depend upon each other for continued viability. We wondered whether the presence of chromosome abnormalities, which are extremely common in human eggs and are incompatible with the formation of healthy embryos, would have an effect on the surrounding cumulus cells. This is important for two reasons.

Firstly, an increase in the understanding of how chromosome abnormalities arise in eggs is desperately needed. For several decades we have been aware that chromosome problems are common in human eggs, that they are the major cause of miscarriage, and that they are responsible for conditions such as Down syndrome. Yet the origins of chromosome abnormalities remain poorly understood. A better understanding of the factors that lead to chromosome abnormality may help us think of ways to reduce their frequency.

Secondly, if chromosome abnormalities in the egg result in changes in the surrounding cumulus cells, it is possible that this could lead to a new way of testing eggs, before they are fertilised, revealing those with the correct number of chromosomes as well as those that are abnormal. This could help patients undergoing IVF, by identifying the eggs most likely to make a baby without having to use an invasive and expensive procedure. Cumulus cells are routinely stripped off eggs during IVF treatments and are usually discarded, so it should be straightforward to obtain them for analysis.”

Cumulus cells and polar bodies (by-products of egg formation that contain discarded chromosomes) from 26 eggs donated by women undergoing PGS were examined by the researchers. Any abnormality in the first polar body results in an abnormality of the corresponding egg, e.g. the presence of an extra chromosome in the polar body indicates an absence of one chromosome in the egg. Scientists identified a total of 13 normal and 13 abnormal eggs by testing the polar bodies.
Dr Fragouli said:

“We then looked to see how active individual genes were in the cumulus cells that had surrounded each egg. This was done using two different methods. First we used a microarray, a powerful genetic technology that allows the activity of thousands of genes to be simultaneously tested. We found that 729 genes were expressed differently in cumulus cells that had surrounded eggs that contained an incorrect number of chromosomes. In other words, these genes were either more or less active than we would usually expect. In particular, 14 genes appeared to have highly significant differences in activity when their corresponding egg was abnormal.

We then used a second technique to confirm the initial findings. For this purpose we focused on 95 of the 729 genes that had been originally identified, including the 14 very significant genes. The method we used is known as real-time polymerase chain reaction (PCR). Real-time PCR is considered to be the most accurate way of quantifying the activity of genes, but is difficult to apply to large numbers of genes, which is why we used the microarray for the initial round of screening. The real-time PCR confirmed that most of the genes highlighted by the microarray do indeed show altered activity in cumulus cells associated with abnormal eggs.

We are still in the process of establishing the usefulness of these genes as non-invasive markers of egg chromosome status and quality. However, it is interesting that several of these genes are involved in vital cellular functions of the cumulus cells and egg they enclose, such as cell signalling and regulation, hormonal response and cell death, and so they may shed light on the genetic origins of chromosome abnormality.”

Scientists are presently conducting more tests to understand how well results from the expression of genes in cumulus cells compare with the more commonly used PGS method for identifying abnormal eggs. A clinical trial may be planned within the next one year if there is evidence of good correlation.

Dr Fragouli concluded:

“The general idea is that instead of manipulating and biopsying the oocyte, a test examining the corresponding cumulus cells, which are currently discarded during regular IVF treatment, is developed. At the moment, as we are still working on this, I would envisage that results would potentially be obtained between three and five hours after egg retrieval. Theoretically, it would be possible to avoid fertilisation of abnormal eggs, which might have some ethical advantages over the current invasive methods that generally take longer. In addition, current diagnostic methods available for preimplantation genetic screening only provide information on the chromosome status of an egg. While this is a very important aspect of egg quality, it is not the only factor influencing the ability of the egg to lead to a successful pregnancy. The extra genetic information that we may be able to derive from examining the cumulus cells may give us a more detailed evaluation of an egg’s potential to lead to a successful pregnancy and a healthy live birth.”

Funded by GEMA Diagnostics (Michigan, USA) and Reprogenetics UK (Oxford, UK).

Written by Anne Hudsmith